As a conventional method for extracting an alkali metal and/or alkali earth metal from a solid containing the alkali metal and/or alkali earth metal, for example, the method disclosed in Patent Document 1 is known. Patent Document 1 discloses a method in which magnesium and calcium are extracted from steel slag or the like by adding steel slag or the like to an aqueous solution containing formic acid or citric acid so as to elute magnesium and calcium, and thereafter injecting carbonic acid gas to the aqueous solution so as to precipitate out as carbonates (magnesium carbonate and calcium carbonate).